Chlorination of manganese oxides

Article abstract of DOI:10.1016/j.tca.2009.05.004

In this work the reaction between several manganese oxides and chlorine is investigated. The reaction path for the chlorination of the oxides is established, which involves recrystallization of high valence manganese oxides: Mn₃O₄ an

Full text of DOI:10.1016/j.tca.2009.05.004

Thermochimica Acta 494 (2009) 141–146
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Chlorination of manganese oxides
Gastón G. Fougaa,b, Georgina De Miccoa,c,d,∗, Ana E. Bohéa,b,c
a Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina
b Universidad Nacional del Comahue, Centro Regional Universitario Bariloche, 8400 San Carlos de Bariloche, Argentina
c Comisión Nacional de Energía Atómica (C.N.E.A.), Avenida Bustillo 9500, 8400 San Carlos de Bariloche, Argentina
d Universidad Nacional de Cuyo, Instituto Balseiro, Avenida Bustillo 9500, 8400 San Carlos de Bariloche, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work the reaction between several manganese oxides and chlorine is investigated. The reaction
path for the chlorination of the oxides is established, which involves recrystallization of high valence
manganese oxides: Mn3O4 and Mn2O3 during the chlorination of MnO, and Mn2O3 during the chlori-
nation of Mn3O4 and Mn2O3. The starting temperature for the reaction of the oxides is determined by
non-isothermal thermogravimetric measurements. Analysis of the samples at different reaction degrees
reveals that three simultaneous processes are taking place during the whole reaction: chlorination,
volatilization of manganese chloride and recrystallization of manganese oxides. The effect of temperature
in the reaction rate is analyzed. The activation energies obtained for the reaction of the three oxides with
chlorine, which are in accordance with the vaporization enthalpy of MnCl2, suggest that although during
the mass loss three processes occur (chlorination, recrystallization and volatilization), volatilization of
manganese chloride has a strong influence in the rate observed.
Received 15 March 2009
Received in revised form 2 May 2009
Accepted 8 May 2009
Available online 20 May 2009
Keywords:
Chlorination
Manganese oxides
Thermogravimetry
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
indicate the possibility of preferential chlorination of Cu, Ni, Co and
Mn with respect to iron.
Many oxide ores which have manganese in their composition
undergo chlorination processes during the extraction of refractory
metals such as Ti, Hf and Nb. For this reason, when these types of
minerals are subject to chlorination, also manganese oxides may be
involved in the reaction. To establish the importance of manganese
elimination process in the chlorination of these minerals, it is nec-
essary to study the chlorination of manganese oxides. The present
study arises from a previous one from Fouga [1] which is about
the chlorination of an ilmenite from Venezuela with the following
composition: Fe0.96Mn0.04TiO3.
Dry chlorination processes can also be applied for recovering
and recycling of valuable metals [2–5]. Recently, a method has been
proposed that involves the addition of chloride that comes from an
undesired waste formed during cement production, to separate and
recuperate metals from bottom ashes by formation of volatile metal
chlorides [6]. In order to develop this kind of process it is necessary
to study the reaction of metal oxides with chlorine. Murthy and
Reddy [7] applied chlorination for the extraction of Cu, Ni, Co, and
Mn from manganese nodules from the Indian Ocean. Their results
The basic advantage of the chlorination process is that the
selective extraction of metals could be affected by exploiting the
difference either in the reactivity of the metal oxides with chlo-
rine or taking advantage of differences in the volatilization and/or
condensation temperatures of their chlorides.
Regarding recuperation of manganese, the results of this study
could also be applied in the development of methods for recupera-
tion of manganese from spent dry-cell batteries.
A few works related with chlorination of manganese oxides were
reported, and there is no information about the kinetics or mecha-
nism of the manganese oxide chlorination reactions.
Okahara and Iwasaki [8] investigated the chlorination of MnO,
Mn3O4, Mn2O3 and MnO2, they obtained the non-isothermal ther-
mogravimetric curves, the starting temperature for the chlorination
ofMnO, Mn3O4 andMn2O3 weredeterminedat200, 350and450 ◦C,
respectively. For the chlorination of Mn3O4 and Mn2O3 they postu-
lated the following reactions:
2Mn2O3 + Cl2 → MnCl2 + 3MnO2
(1)
(2)
Mn3O4 + Cl2 → MnCl2 + 2MnO2
For the initial state of MnO chlorination they proposed:
MnO + Cl2 → MnCl2 + MnxOy
∗
Corresponding author at: Consejo Nacional de Investigaciones Científicas y Téc-
nicas (CONICET), Argentina. Tel.: +54 2944 445293; fax: +54 2944 445293.
(3)
E-mail address: demiccog@cab.cnea.gov.ar (G. De Micco).
0040-6031/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2009.05.004

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G.G. Fouga et al. / Thermochimica Acta 494 (2009) 141–146
Ivashentsev and Simkina [9] determined that the chlorination of
MnO by Cl2 begins at 130 ◦C and occurs up to 310 ◦C according to:
MnO(s) + Cl2(g) → MnCl2(l) + (1/2)O2(g),
ꢀG◦ = 0.0058 T(◦C) − 46.9
(7)
(8)
MnO + Cl2 → MnCl2 + (1/2)O2
(4)
Between 310 and 400 ◦C, Mn2O3 was detected in the products
and it also reacts with Cl2 according to:
(2/3)MnO(s) + Cl2(g) → (2/3)MnCl3(g) + (1/3)O2(g),
ꢀG◦ = 0.034 T(◦C) + 45.96
2Mn2O3 + 4Cl2 → 4MnCl2 + 3O2
(5)
They found that formation of MnCl2 starts at 300 ◦C for Mn3O4
(1/2)MnO(s) + Cl2(g) → (1/2)MnCl4(g) + (1/4)O2(g),
ꢀG◦ = 0.0136 T(◦C) + 49.86
and at 430 ◦C for Mn2O3.
(9)
In the present work the starting temperatures for the chlo-
rination of manganese oxides were obtained, since there is no
good agreement in the values reported as mentioned before. The
reactions occurring during manganese oxide chlorination were
identified and formation of high valence manganese oxide was
determined.
MnO(s) + Cl2(g) → (1/2)Mn2Cl4(g) + (1/2)O2(g),
ꢀG◦ = −0.033 T(◦C) + 26.2
(10)
Among these, formation of MnCl2 and Mn2Cl4 are the only ones
that have negative ꢀG◦ values in the range of temperatures con-
sidered. The dimerization reaction (Eq. (11)) is thermodynamically
feasible below 870 ◦C.
2. Experimental
The gases used were Cl2 99.8% purity (Indupa, Argentina) and
Ar 99.99% purity (AGA, Argentina). The solids were MnO powder
(reagent grade, Alfa Aesar Company), and Mn3O4 and Mn2O3 which
were obtained from MnO by thermal treatments in air for 24 h at
950 and 600 ◦C, respectively.
The powder samples of each oxide were characterized by
scanning electron microscopy (SEM 515, Philips Electronics Instru-
ments) and X-ray diffraction (XRD). XRD patterns obtained
(Fig. 1a–c), are in good agreement with the corresponding refer-
ence patterns for the oxides [10] also shown in the figure. No other
phases were detected in each pattern.
Fig. 1d and e shows the morphology of MnO, it consists of inter-
growth crystals of different sizes ranging between 20 and 80 ␮m.
Micron-order pores were observed in the crystal surface by SEM
(Fig. 1e). A BET area [11] of 0.53 m2/g was determined with N2
(Digisorb 2600, Micrometrics Ins. Corp.) for the initial MnO pow-
der. The morphology of the Mn3O4 and Mn2O3 samples (Fig. 1f–i)
is similar to that of MnO. The high valence oxides are formed by
well-shaped crystal agglomerates greater than 100 ␮m in size. Also
a decrease in the surface porosity was observed.
Isothermal and non-isothermal chlorination reactions were car-
riedoutinathermogravimetricanalyzer(TGA), whichisextensively
described elsewhere [12]. This thermogravimetric analyzer consists
of an electrobalance (Model 2000, Cahn Instruments, Inc.) suitable
for working with corrosive atmospheres, a gas line and an acqui-
sition system. This experimental set-up has a sensitivity of 5 ␮g
while operating at 950 ◦C under a flow of 8 l/h. Samples of 20 mg
were placed in a quartz crucible inside the reactor in an argon flow
of 1.3 l/h. To begin the non-isothermal reactions, a chlorine flow
of 0.8 l/h was introduced and, at the same time, the heating started
with a ramp of 100 C/h. For the isothermal reactions, the solids were
heateduntiltheyreachedthedesirabletemperaturebeforechlorine
injection. The partial pressure of chlorine in the Ar–Cl2 mixture was
35.5 kPa.
2MnCl2(g) → Mn2Cl4(g),
ꢀG◦ = 0.14 T(◦C) − 122.17
(11)
The experimental set-up used allows continuous evacuation of
the reaction products (i.e. O2, MnCl2), leading to low partial pres-
sures of these species, whereas partial pressure of Cl2, which is
given by the incoming chlorine stream, will remain constant at
35.5 kPa. For this reason, the values of ꢀG will be lower than the
values of ꢀG◦, and even with a ꢀG◦ > 0 a reaction could occur.
On the other hand, an opposite situation can be held locally at
certain places. Although the chlorinating gas is a mixture of Cl2–Ar,
in the sample surroundings O2 is being generated and Cl2 is being
exhausted by the reaction, for that reason the partial pressure of O2
is expected to be high. Moreover, the residence time of O2 in the
sample is long due to the low gaseous flow (2.1 l/h) and because
the O2 formed may have to diffuse through a MnCl2 layer before
reaching the gaseous phase. A Kellog-diagram [13] (Fig. 2) of the
Mn–Cl–O system at 800 ◦C shows the stability areas of manganese
containing substances as a function of Cl2(g)- and O2(g)-pressures
in the atmosphere at that temperature. It shows that for partial
pressures of Cl2 close to zero value, as expected at the reaction sur-
face, partial pressures of O2 lower than 101.3 kPa (1 bar) are high
enough to allow formation of high valence manganese oxides such
as Mn3O4 and Mn2O3.
Finally, no information is available regarding manganese oxy-
chlorides, with the only exception of the existence of gaseous
MnClO3 that has been reported [13].
4. Results and discussion
4.1. Reactivity
The initial reaction temperature which is associated with the
system reactivity was determined by non-isothermal thermogravi-
metric measurements. The chlorination curves for MnO, Mn3O4 and
Mn2O3 are shown in Fig. 3. This figure shows the ratio between the
mass change and the initial mass of the sample (ꢀM/Mi) as a func-
tion of temperature. The starting temperatures for the chlorination
of MnO, Mn3O4 and Mn2O3 are 270, 380 and 520 ◦C, respectively.
The TG curves start with a slow mass gain which corresponds
to formation of condensed MnCl2, and high valence oxides, as will
be discussed later. When temperature reaches about 650 ◦C (MnCl2
melting point) there is an increase in the reaction rate, this behav-
ior may be due to the melting of the chloride layer formed that
allows chlorine to easily reach fresh oxide. The mass gain observed
in the maximum of the curves for the three systems are lower than
3. Thermodynamical analysis
Several manganese chlorides are reported, MnCl, MnCl2, MnCl3,
MnCl4 and Mn2Cl4. The values of ꢀG◦ (kJ/mol of Cl2) as a func-
tion of temperature, corresponding to formation of these chlorides
are presented in Eqs. (6)–(10) by mol of chlorine for temperatures
between 650 and 950 ◦C:
2MnO(s) + Cl2(g) → 2MnCl(g) + O2(g),
ꢀG◦ = −0.344 T(◦C) + 689.67
(6)

G.G. Fouga et al. / Thermochimica Acta 494 (2009) 141–146
143
Fig. 1. XRD patterns, SEM photographs of the initial samples of MnO, Mn3O4 and Mn2O3.
that needed for complete reaction to form MnCl2 which indicates
until complete volatilization.
that also volatilization of the chloride formed is taking place. This
is expectable since the partial pressure of manganese chloride (also
shown in Fig. 3) reaches a value of 10−2 kPa at 640 ◦C [13], which
suggests that the mass change due to volatilization will be signif-
icant above that temperature [14]. For temperatures above 730 ◦C
the mass continuously decreases until complete volatilization.
In order to understand how the chlorination reaction occurs,
the chlorinations were interrupted at different reaction degrees (as
shown by symbols in Fig. 4). Samples were analyzed by SEM and
XRD. SEM photographs and phases detected by XRD are shown in
Fig. 4. In the maximum of the curve, after the mass gain, there is
MnCl2, Mn3O4 and Mn2O3. The occurrence of Mn3O4 and Mn2O3
indicates that MnO is being chlorinated according to:
4.2. Isothermal chlorination of manganese oxides
4MnO(s) + Cl2(g) → MnCl2(l) + Mn3O4(s)
3Mn3O4(s) + Cl2(g) → MnCl2(l) + 4Mn2O3(s)
(12)
(13)
The TG curve for the chlorination of MnO at 800 ◦C is shown in
Fig. 4. The reaction starts with a mass gain followed by a mass loss

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G.G. Fouga et al. / Thermochimica Acta 494 (2009) 141–146
Fig. 3. Non-isothermal chlorination reactions of MnO, Mn3O4 and Mn2O3.
Fig. 2. Predominance diagram for Mn–Cl–O system at 800 ◦C (partial pressures
expressed in bar units). The average reaction conditions during chlorinations are
indicated by
.
To check the occurrence of the reactions proposed, chlorinations
of Mn3O4 and Mn2O3 at 800 ◦C were performed, and interrupted
reaction samples were analyzed. The corresponding TG curves are
shown in Fig. 5. Both reactions were interrupted during the mass
loss near the maximum, and Mn2O3 and MnCl2 were detected by
XRD. Regarding Mn2O3 chlorination, an interrupted reaction shows
that the sample consists of MnCl2 and Mn2O3.
Formation of MnCl2 during the chlorination of manganese
tri-oxide in a highly oxidizing atmosphere can be understood con-
sidering the Kellog-diagram (Fig. 2). With a simple calculation
based on the moles of oxygen lost during the chlorination of Mn2O3
it is possible to estimate the average partial pressure of O2 under
the experimental conditions, the value is in the order of 0.1 kPa
(10−3 bar). The partial pressure of Cl2 is given by the incoming chlo-
rine stream (35.5 kPa = 0.355 bar). A symbol was included in Fig. 2
which indicates these conditions, it can be seen that the stability
diagram predicts formation of MnCl2.
The high ratio of mass gain obtained in the maximum of the
curve, 0.29 which is higher than the one expected taking into
account Eq. (12) (i.e. 0.26), is in accordance with the reactions pro-
posed.
SEM examination shows that while manganese chloride forms
agglomerates of small grains (Fig. 4a), manganese oxides growth
as well defined crystals (Fig. 4b). The morphology of the oxides
indicates that they are being formed from an isotropic media, two
possible processes can lead to formation of these crystals: a gaseous
phase reaction taking place in the boundary layer between man-
ganese chloride and oxygen which is thermodynamically feasible,
or a reaction occurring in the liquid phase formed by liquid man-
ganese chloride saturated with dissolved oxygen.
An advanced stage of the reaction was analyzed by interrupting
a chlorination during the mass loss (also shown in Fig. 4.) MnCl2 and
Mn2O3 were detected by XRD the corresponding morphologies are
shown in Fig. 4d and e. These results are also in accordance with
reaction (13).
These results confirm that chlorination of Mn3O4 occurs through
formation of Mn2O3 (Eq. (13)) and chlorination of Mn2O3 accord-
ing to Eq. (14) with formation of manganese chloride and oxygen
Fig. 4. Chlorination of MnO at 800 ◦C.

G.G. Fouga et al. / Thermochimica Acta 494 (2009) 141–146
145
Fig. 5. Chlorination of Mn3O4 and Mn2O3 at 800 ◦C.
release:
(1/2)Mn2O3(s) + Cl2(g) → MnCl2(s, l) + (3/4)O2(g)
(14)
To sum up, the following reaction mechanism for the chlori-
nation of MnO can be proposed: chlorination of manganese oxide
with formation of manganese chloride and manganese tetra oxide
according to Eq. (12), chlorination of manganese tetra-oxide with
formation of manganese chloride and manganese tri-oxide accord-
ing to Eq. (13) and chlorination of manganese tri-oxide according
to Eq. (14).
Consequently, during the chlorination of manganese oxides at
least three different processes can influence the rate observed:
chlorination, volatilization and recrystallization.
4.3. Influence of temperature in the chlorination of manganese
oxides
Isothermal chlorinations of the different oxides in the range
between 650 and 950 ◦C and chlorine partial pressure of 35.5 kPa
were performed. The influence of temperature on the chlorination
reactions can be observed in Fig. 6a–c, where the TG curves for the
chlorination of MnO, Mn3O4 and Mn2O3 are presented.
These curves show two different regions, an initial mass gain
where the rate of formation of condensed manganese chloride and
high valance manganese oxides is higher than the rate of volatiliza-
tion, and a region of mass loss where the rate of volatilization of the
chloride formed predominates. At high temperatures only weight
losses were observed although the conversion of the oxides into
MnCl2 is characterized by a significant weight gain, this is because
the rate of volatilization is so enhanced by temperature that the
amount of mass loss due to volatilization is higher that the amount
of mass gain due to formation of manganese chloride leading to an
overall mass loss.
Fig. 6. Influence of temperature in the chlorination of manganese oxides.
ary layer controls the reaction rate as demonstrated by previous
publication [15].
The rate of mass loss increases with temperature, the values
obtained for the different oxides are shown in Table 1.
These values are lower than the aforementioned values
(10−6 molesofCl2/si.e. 7 × 10−2 mg/s), whichindicatesthatfortem-
peratures below 950 ◦C, the rate is not controlled by diffusion of the
incoming chlorine through the boundary layer. The constant values
obtained for the different oxides suggest that the rate is controlled
The initial rate of the mass gain is independent of temperature
and the rate order, 10−6 moles of Cl2/s, reveals that the chlorination
reaction is very fast and diffusion of chlorine through the bound-
Table 1
Rate of mass loss in mg/s during manganese oxide chlorination.
Oxide
Temperature in ◦
700
C
750
800
850
900
950
MnO
Mn3O4
Mn2O3
1.2 × 10−3
8.3 × 10−4
8.8 × 10−4
2 × 10−3
–
3.9 × 10−3
4.9 × 10−3
3.6 × 10−3
9.5 × 10−3
1.1 × 10−2
8.4 × 10−3
2.2 × 10−2
2.2 × 10−2
2.2 × 10−2
4.6 × 10−2
4.2 × 10−2
4.9 × 10−2
2 × 10−3

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G.G. Fouga et al. / Thermochimica Acta 494 (2009) 141–146
by volatilization of MnCl2. Activation energies of 171 9 kJ/mol,
148 3 kJ/mol and 173 8 kJ/mol were calculated for the reactions
of MnO, Mn3O4, and Mn2O3 with Cl2, respectively, with the Flynn
method [16] from the slope of the curve of ln t versus 1/T during the
mass loss. These values are similar to the average value of the vapor-
ization enthalpy of manganese chloride between 750 and 950 ◦C,
162 kJ/mol [13], which also suggests that volatilization of MnCl2
has a strong influence in the rate of reaction during the mass loss.
However, a detailed kinetic study is beyond the scope of the present
work.
Acknowledgements
The authors would like to thanks the Agencia Nacional de Pro-
moción Científica y Tecnológica (ANPCyT), Consejo Nacional de
Investigaciones Científicas y Técnicas (CONICET) and Universidad
Nacional del Comahue for the financial support of this work.
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5. Conclusions
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